Heart rate and force of contraction are under antagonistic adrenergic (excitatory) and cholinergic (inhibitory) control, mediated by the neurohormones norepinephrine and acetylcholine, respectively. These agents exert their action by binding to specific membrane receptors: beta-adrenergic and muscarinic-cholinergic. Using a newly-developed tissue-culture system in which single isolated mouse cardiocytes as well as spontaneously formed clusters of cardiocytes respond in a physiologic fashion to both autonomic neurohormones, and which can be innervated at will by intrinsic cholinergic neurons, I will study: (1) regulation of the number, distribution, and pharmacologic and kinetic properties of the receptors as a function of cardiocyte-cardiocyte, cardiocyte-neuron, and cardiocyte-neurohormone interactions; and (2) regulation of the response of the cardiocytes upon simultaneous stimulation of both receptor systems. The study will use a parallel electrophysiologic, pharmacologic and radioligand binding approach, to (1) define the properties of the receptor(s)' binding site(s) and (2) to explore how these and/or the receptor(s)' coupling to ionic channels which underlie automaticity are modified by homotypic or heterotypic cell-cell interactions.